1. Field of the Invention
The present invention relates to an optical recording medium and a fabrication method thereof, and to a near-field recording/reproducing device for recording/reproducing data on/from the optical recording medium, and to a method for, and in particular to a recording medium for recording/reproducing information by using an evanescent field transmitting an electromagnetic interaction controlled in the visible light and infrared regions, overcoming a diffraction limit, and having an ultra-high recording density, and a fabrication method thereof, and to a near-field optical recording/reproducing device for recording/reproducing information on the recording medium, and a method therefor.
2. Description of the Background Art
As the multimedia era in which information of various types, such as audio and video motion picture and text files are integrated has begun, there is an increasing demand for information recording and storage media having a large capacity which can rapidly process and store a large amount of information. Especially, if a two-way picture communication, such as a high definition motion picture and a video-on-demand (VOD) which are expected to be widely distributed is to be realized, the information recording and storage media must be increased in capacity. In order to meet the demand, various recording/reproducing methods have been suggested for the commonly-used recording media, and a magnetic recording method and an optical recording method are commonly used among the methods.
A prime example of the magnetic recording method is a method of recording/reproducing information on a hard disk by using a hard disk drive (HDD).
In order to store more information on the hard disk, the recording density of the hard disk should be improved. For this, a flying height between a head and a recording medium must be lowered. Here, in order to reduce the flying height, a lower portion of the head must be located very closely to the recording medium. However, when the lower portion of the head approaches to closely to a surface of the recording medium, the head scratches it, so called xe2x80x9ca head crashxe2x80x9d takes place, thereby damaging the recording medium and reducing the reliability thereof.
In addition to the method of improving the recording density by lowering the flying height between the head and the recording medium, there is a method of reducing a size of a MR-head (magnetic-resistive head) in the hard disk drive.
In the case of a longitudinal magnetic recording medium which has been popularly used, if the size of the particles composing each magnetic domain is lesser than a critical size, a recording bit causes magnetic inversion due to thermal fluctuation in an environment of a normal temperature. As a result, the recording medium cannot be well operated. This phenomenon is called a xe2x80x9csuper-paramagnetic limitxe2x80x9d. It is thus impossible to record and retain information on the magnetic disk at an ultra-high recording density exceeding the limit. If the study for the high recording density of the magnetic recording medium proceeds according to the current tendency, it is expected that a limit value thereof will reach to 40 Gbit/in2 around 2005.
On the other hand, the optical recording method is a method of recording/reproducing information on a recording medium, an optical pickup not being contacted with the recording medium. It does not have such a problem as the head crash occurring in the magnetic recording method. Accordingly, an optical recording medium such as a compact disk (CD) using the optical recording method can partially replace the magnetic recording media such as a magnetic tape and a magnetic disk using the magnetic recording method. In addition, the optical recording medium according to the optical recording method can be easily installed in the optical recording device using the optical recording method, and is convenient to carry. For instance, even if a CD drive is operated in a moving vehicle, a head of the optical recording device and the optical recording medium do not physically damage each other, and thus the information recorded on the optical recording medium is retrieved in safe manner. Also, the optical recording medium has a narrow track pitch, as compared with the magnetic recording medium, thereby achieving the higher recording density.
Nevertheless, a storage amount of the CD which has been widely distributed is only 650 megabytes. Accordingly, it is too small to deal with a large amount of information such as motion picture. In addition, in the case of a digital versatile disk (DVD) which has been recently commercialized, a recording capacity thereof is seven times as much as that of the CD (4.7 GB). However, a new recording medium having a larger capacity than DVD is required in order to freely deal with the motion pictures like movies. Thus, many studies have been made in association with the ultra-high recording density of the recording medium.
The ultra-high recording density will now be described exemplifying optical recording media, such as a read only memory (ROM) and a rewritable memory (-RAM/RW). The most important factor influencing the recording density of the optical recording media is a spot size of a laser light beam. That is, the smaller the spot of the laser beam is, the more information we can record in the optical recording medium. For this, the wavelength of a laser light source must be shortened, and a numerical aperture (NA) of an objective lens of the optical pickup must be increased.
However, although the wavelength of the laser light source is shortened and the numerical aperture of the objective lens is increased, the size of the spot of the laser beam can be narrowed merely to the extent of the wavelength of the laser source. For example, in order to shorten the wavelength of the laser light source, when a laser diode having a wavelength of 400 nm is used as a light source of the digital versatile disk DVD, instead of a laser diode having a wavelength of 650 nm which has been used for the DVD, the amount of information recorded per unit area of the DVD can be approximately 2.5 times as much as an amount of the information of the recording medium when the laser diode having the wavelength of 650 nm is used.
However, even if the short wavelength laser diode is employed as a light source for recording information on the optical recording media, such as the CD and the DVD, there is still a recording limit. In addition, it is impossible to improve the recording density of the recording media over a predetermined limit by overcoming the diffraction limit, due to a property of the diffraction resulting from the wave-nature of the light.
Accordingly, in order to process information of a terabyte, the ultra-high recording methods such as a near-field optics, a volume hologram, a photo-chemical hole burning and a three-dimensional optical recording are suggested on the basis of a totally different principle from the conventional art.
However, in the cases of the volume hologram and the photo-chemical hole burning, there is a remarkable limit in an environment of using the recording medium. Practically, the optical recording medium using the near-field cannot be easily used.
The background of inventing the near-field method will now be explained.
In general, in a diffraction theory, the spot size (diameter) of a focused light is determined by the optical wavelength and the numerical aperture of the objective lens. An upper limit of the recording density of the recording medium is determined by how small the light focusing spot is formed. According to the diffraction phenomenon, as a beam size of the light is reduced by using a lens, the beam is more widened. It is represented by the following expression (1).                     θ        =                                            2              ⁢                              xe2x80x83                            ⁢              λ                                      π              ⁢                              xe2x80x83                            ⁢              d                                ≅                      λ            d                                              (        1        )            
Here, xe2x80x9cxcex8xe2x80x9d, xe2x80x9cdxe2x80x9d and xe2x80x9cxcexxe2x80x9d indicate the diffraction angle, the waist of the beam, and the wavelength of the light, respectively. According to the diffraction theory, if the size of the beam is reduced by using the lens, the diffraction angle is increased. In addition, the beam size cannot be less than a predetermined value originated from the diffraction limit.
Therefore, a limit of the recording density of the optical recording medium is governed by the diffraction theory of light which is approximately represented by the following expression (2).                     d        ≅                              1.22            ⁢                          xe2x80x83                        ⁢            λ                    NA                                    (        2        )            
Here, xe2x80x9cdxe2x80x9d, xe2x80x9cxcexxe2x80x9d and xe2x80x9cNAxe2x80x9d indicate the waist of the beam, the wavelength, and numerical aperture of an objective lens. That is, the shorter the wavelength (xcex) of the laser light source is, and the more the numerical aperture of the lens NA is increased, the smaller the focused beam size is. The recording density of the recording medium is increased in an inverse proportion to the square of the spot size. A minimum size of the information which can be recorded/reproduced per bit approximately corresponds to the light wavelength because of the diffraction resulting from the wave-nature of light.
Accordingly, it is the best method in the conventional art to increase the recording density by shortening the light wavelength, and by decreasing the size of the focused spot with the lens having large aperture. It is expected that the recording density which may be obtained by this method is between 20 and 30 Gbit/in2. Thus, the conventional system has a disadvantage in that the light is used as an electromagnetic wave, and thus there is a limit resulting from the diffraction in improving the recording density.
In order to overcome the diffraction limit, a light existing in an evanescent field of a near-field region (i.e., a distance less than the light wavelength from a material surface) is employed on the recording medium. That is, a concept that the light (an evanescent light around the aperture) is not emitted from an aperture shorter than the light wavelength, but interacts with a material positioned around the aperture, is applied to recording/reproducing of the information on the recording medium, thereby overcoming the diffraction limit.
For this, in the conventional art, after an optical fiber is sharpened, and a metal film (reflective film) such as aluminum is deposited, the metal film at an edge portion of the optical fiber is removed, thereby forming an aperture smaller than the light wavelength. The optical fiber is used as a probe. When the light is incident on the end of the optical fiber, the light having a normal wavelength is not emitted from the probe, and only the evanescent light is obtained.
Consequently, the aperture formed by the optical fiber in the conventional art overcomes the diffraction limit, and improves the recording density of the optical recording media. That is, the above-described method employs the evanescent light in order to prevent a phenomenon of widening the spot due to the light diffraction, and to reduce the spot size to less than the light diffraction limit.
Here, the evanescent light is a local interaction between polarizations generated in a material due to the irradiation of light. When the edge portion of the optical fiber is sharpened, and injected around a sample surface, the resolution thereof is determined by the sharpness of the edge portion of the optical fiber probe, thereby overcoming the diffraction limit of the light. In this case, a size (diameter) of the evanescent light beam is too small (equal to a size of the edge portion of the optical fiber) to be made by using an optical device such as a lens causing the diffraction phenomenon.
However, in the recording/reproducing process, it is necessary to locate the probe by as close as a dimension of the size of the end portion of the probe to the optical recording medium having a dimension of a particle size corresponding to a dimension of the size of the end portion of the probe, and to inject the probe in such state. A nanometer technology is required to embody this system.
As described above, the method used in the system for recording/reproducing the information on the recording medium by using the near-field generally employs the optical fiber as the probe, the end portion of which being sharpened at its edge portion, covered with the metal film, and provided with an aperture much smaller than the light wavelength. The light emitting from the probe is not diffracted in the near-field region, and can form very small spots. Thus, it can be utilized to observe a material surface or a physical phenomenon, and to record/reproduce the high-density information.
Here, a continuous servo is required to bring the recording medium in the near-field region. In case an external impact is applied or the material surface is uneven in the recording/reproducing process, the optical fiber tip may be damaged.
On the other hand, a scanning near-field optical microscopy SNOM used for obtaining high resolution is applied to the recording medium for recording the information. Here, the SNOM has a disadvantage in that an output signal is very weak. As the near-field light locally existing around micro-apertures is used to record/reproduce the information on the recording medium, the reproducing signal amount is very small(xcx9c10 nW). Accordingly, if the data transfer rate is increased (for example, to 10 MHz or more), the signal-to-noise ratio is reduced.
Here, the SNOM requires the development of a control technique of constantly maintaining the optical fiber from the surface of the recording medium by a gap less than or equal to 10 nm, a precise control servo-technique in the media plane, and a beam high speed scanning method for achieving a high data transfer rate. In addition, a first step which should be taken for recording the information on the recording medium by using the evanescent light is, as described above, to develop a highly sensitive optical recording medium which can react to a small amount of energy because the throughput of the micro-aperture is relatively low.
As discussed earlier, in principle, the information can be recorded and reproduced on the recording medium at a high recording density by using the near-field. However, when the optical recording medium is fabricated according to the conventional method, there are several disadvantages, as follows:
(a) A precise gap control must be performed in order to constantly maintain a distance between the probe and the recording medium surface (xcx9ca few tens nm). It requires a complicated device including a plurality of components, such as a dithering piezoelectric device (PZT), a laser diode (LD), a collimating lens, an optical detector, an amplifier, a lock-in amplifier and a proportional integrator (PI).
(b) It is difficult to process the optical fiber tip to be used for minimizing the spot size of the laser light source. A small number of the fiber tips and apertures having a uniform size can be fabricated in a place such as a laboratory. However, this method is not appropriate for mass production.
(c) The relative movement of recording media over a optical pickup is so rapid that the probe and the recording medium including the information can be easily damaged. The recording device recording and reproducing the information on the recording medium having a high recording density should rapidly approach a position where the wanted information is located. For this, a relative moving speed of the recording media surface and the probe must be high. However, when moved, the probe may collide with the surface of the recording medium. In case the probe is damaged by the collision, it influences on the recording and reproducing characteristics, thereby reducing the reliability of the recording medium.
(d) When a rewritable medium is used for the near-field optical recording, the rewritable medium surface is externally exposed in order to position the recording medium within the near-field region, differently from the CD method. Consequently, the surface of the medium may be contaminated and scratched.
Accordingly, it is a primary object of the present invention to provide a recording medium of a ultra-high recording density having a planar aperture array.
It is another object of the present invention to provide a method for fabricating a recording medium of an ultra-high recording density having a planar aperture array.
It is still another object of the present invention to provide an optical recording/reproducing device which can record and reproduce information on a recording medium of an ultra-high recording density by using a near-field optics.
It is still another object of the present invention to provide a method for controlling an optical recording/reproducing device which can record and reproduce information on a recording medium of an ultra-high recording density by using a near-field.
In order to achieve the above-described objects of the present invention, there is provided a recording medium including a planar aperture array generating a near-field light by an incident laser light, and a recording layer positioned separately from the planar aperture array by a predetermined gap. Here, a size of the aperture of the planar aperture array is smaller than a wavelength of the laser light source so that the aperture can receive the laser light source and generate the near-field light. The gap between the planar aperture array and the recording layer is determined to be in the near-field. The gap is constantly maintained by spacer having a predetermined dimension, and sealed up with a flexible material, and thus inner portions of the planar aperture array and the recording layer are protected from an external environment. In addition, the recording medium further includes a piezoelectric device. The piezoelectric device is driven by a micro-servo, and thus the planar aperture array and the recording layer can be relatively horizontally moved. The recording layer may or may not include a reflection layer, and thus can record and reproduce the information at the both sides or one side thereof.
There is also provided a method for fabricating optical recording media including: a step of fabricating a multi-layer recording media on a first substrate; a step of fabrication of a planar aperture array on a second substrate; and a step of bonding the first substrate and the second substrate having a predetermined gap. Especially, the step of forming the planar aperture array includes a step of forming a film by sputter deposition of a reflective layer on the second substrate; a step of coating a photo-resist (PR) on the reflection layer; a step of forming a pattern of the planar aperture array on the PR coating by using an electron beam; and a step of etching the planar aperture array pattern in an etching solution. In addition, it further includes a step of sealing up the gap between the first substrate and the second substrate in order to be protected from an external environment.
In the method for fabricating the recording medium in accordance with a preferred embodiment of the present invention, the step for forming the planar aperture array can be performed by a plastic forming process.
There is also provided a recording/reproducing device including: a pickup unit generating a near-field light at aperture in the recording media having a planar aperture array and a recording layer; a piezoelectric device PZT included in the pickup unit; a driving unit of the PZT; and a signal processing unit recording and reproducing information on the recording medium. Especially, the PZT driving unit operates the PZT included in the pickup unit, thereby carrying out a macro-servo in order for the pickup unit to store/reproduce information at a predetermined position of the recording medium.
There is also provided a recording/reproducing method of a recording medium including: a step of detecting an address of information to be recorded or reproduced at a predetermined position of the recording medium; a step of moving a pickup unit according to the detected address; and a step of recording/reproducing on the recording medium a signal inputted/detected from the pickup unit moved to the predetermined position.